This invention relates to a method and apparatus for continuous freeze drying, and more particularly to a method and apparatus utilizing solid chemical desiccants, known as molecular sieves, for this purpose.
Freeze drying is a well established process for drying, and thereby preserving, pharmaceuticals such as antibiotics, vitamin preparations, vaccines, etc. Aqueous products to be dried are first frozen solidly, and are then subjected to a high vacuum and a controlled heat input. Under these conditions the water content of the product goes directly from ice to water vapor, by-passing the intermediary liquid phase. This large quantity of sublimating water vapor cannot be pumped out of the system directly by oil sealed rotary vacuum pumps, since the oil therein will quickly become contaminated by water condensing in the oil, resulting in too high a pressure for satisfactory operation. To prevent this from occurring, a refrigerated condenser, connected between the drying chamber and the vacuum pump, freezes out the water vapor on coils, cold surfaces, etc. Alternatively, a chemical desiccant can be used to sequester the sublimating water vapor.
It would be desirable to have a freeze dry apparatus work on a continuous basis, and a number of such devices have been disclosed as is evident in U.S. Pat. No. 3,516,170 of June 23, 1970.
However, continuous freeze dryers which rely on refrigerants to sequester the sublimating water vapor have certain inherent problems. In order for freeze drying to be a continuous process, at least two water vapor condensers are required. While one is operating to collect water vapor, the second must be defrosted, the water drained therefrom, and then it must be refrigerated again to a low temperature before it is reconnected to the vacuum system. This involves complicated and expensive automated refrigeration and valving equipment. If single stage refrigeration compressors are employed to maintain the condensers at the extremely low temperatures of -40.degree. C. or even substantially lower temperatures, these compressors are operating at low efficiency. Often to achieve these low temperatures the compressors are operated at high compression ratios. Under these conditions the compressors are expensive to operate and tend to break down prematurely. Attempts to overcome this problem with cascade refrigeration systems, exotic refrigerants, etc. greatly add to the cost of the original equipment, and add additional operating costs, while often exacerbating the problem of equipment unreliability.
Solid, regenerable chemical desiccants including molecular sieves, have been suggested as a means for sequestering water vapor during freeze drying as is evident from U.S. Pat. No. 3,453,741 of July 8, 1969. Although molecular sieves are high capacity water adsorbing desiccants, and can be repeatedly regenerated, their use in a continuous freeze dryer presents a number of difficulties. Molecular sieves are good insulating materials with heats of adsorption of approximately 1,800 B. T. U.'s per pound of water adsorbed. This means that during a vacuum drying procedure they will heat up rapidly. And as they heat up their ability to adsorb moisture under vacuum conditions rapidly declines.
Also for reasons relating to thermal insulation molecular sieves must be regenerated at high temperatures, normally requiring a heated purge gas to remove the moisture.
And any granular, solid desiccant such as molecular sieves must be packed so that water vapor is efficiently adsorbed and non-adsorbable gases permitted high flow rates through the sieve so that vapor flow restrictions are not so high as to induce melting of the product being freeze dried.
Accordingly it is an object of the invention to provide a practical continuous freeze dryer.
Another object is to provide a simple, efficient, non-refrigerated means for sequestering sublimating water vapor during continuous freeze drying.
Another object of the invention is to provide a continuous molecular sieve freeze dryer.
Another object is to provide a simple means for controlling the exotherm which builds up in the molecular sieve during freeze drying.
A further object is to provide a means for regenerating a molecular sieve in a continuous freeze dryer without using a purge gas.
And finally it is an object of this invention to provide a molecular sieve freeze dryer that can adsorb water vapor at a high rate, and be vacuum evacuated rapidly, so that continuous freeze drying can occur on an efficient basis.